Dry Rot

The only so-called dry rot fungus is a particular species of brown rot. The affected wood is brown and causes cuboidal cracking, these "cubes" are usually 25mm² or more and tend to be larger than those of other brown rots.

Records as early as 1700 details the fungus as Merulius Lacrymans, " Tears of the black bird". Over the years, infected timbers have been treated and replaced where possible and it is unknown how many properties have been lost or demolished as a result of these infections.

In 1820 the rot became more commonly known as Serpula Lacrymans "Tears of the snake" as a result of its snake like growth. The Fungi forms tear like droplets of water hanging from the under side

Dry rot is often referred to and becoming more commonly known as building cancer, due to it's devastatingly destructive affect on timber with the ability to travel several metres through masonry in the search for further cellulose material (food). It is also able to transport moisture to dryer timber.

The fungus has a unique ability as it is able to transport moisture from source and infect further timber that may be just below the require moisture levels. This makes it a very pervasive type of rot. Dry rot mycelium and strands are often found growing at the interface between brick and lime sand plaster. They may also be found in the mortar joints.

This growth can only take place through relatively old lime mortars and lime plasters where the alkalinity has become reduced over time. High alkalinity, as found in fresh lime or cement mortar, will normally prevent growth through the mortar joints, although growth over the surface of new brickwork is possible.

Dry rot life cycle

Requires moisture to begin, wood being hygroscopic will absorb moisture from materials it is in contact with and high humidities similar to that which aid Aspergillus like moulds. Bacteria and micro fungi colonise resulting in partial breakdown of the cell structure, the timber becomes more porous and wetter. Timber with moisture sustained above 30%mwe provides the perfect environment for dry rot which can to colonise and germinate leading to decay of timber.

Spores land on cellulose materiel, germinate and produce hyphae growth (this is the starting process and the rooting structure). Each hyphae releases chemicals (enzymes) into the surrounding wood. Decay is caused by the process of these chemicals dissolving nutrients in the wood. The nutrients are then absorbed by the fungal hyphae, enabling further growth.

Mass colonisation will result in mycelium, which is the white sheet like growth often found on the underside of suspended floors. Mycelium can hold small water droplets. After time dry rot becomes stressed from one of the following:-

Lack of water (moisture)

air

lack of food (cellulose materiel like timber)

exposure to light

a disturbance to its habitat

The rot will form a sporophate, which is a fruiting body. They can appear on infected masonry surfaces, often in positions apparently remote from the decaying timber and can send out thousands of spores as the mould reproduces.

Fungal growth can be rampant in the right conditions and has been known to travel 2m per year and up to 4m in lavatory conditions. It generally needs severe water ingress to start off, such as leaking gutters, cracked down pipes, or rising damp etc.

Damage

Wood loses its strength and weight and breaks in to cubical pieces. Larger sectional timbers can appear sound to the surface, but this may only be a 2-5mm veneer where the infection has travelled through the inner core of the timber. Special attention will be required for large sectional timers within contact of other timbers which are infected with this fungus. It is likely that engineers reports will be required.

Dry rot forms

Dry rot spores are omnipresent (in the atmosphere all the time). As the spores are air born all cellulose materiel is susceptible to attack given the right conditions.

Dry rot fungi call for special consideration when estimating remedial works. As they have a higher tolerance to alkaline conditions then wet rot, this allows them to spread through the pores of masonry especially in older properties. The fungi derives no nutrition from masonry and causes no significant damage to the masonry itself. However, the fungi is able to spread relatively rapidly from an initial outbreak through damp masonry in search for further damp timber in contact with the wall. For this reason prompt remedial action is required after the discovery of dry rot.

Dry rot growth through the masonry can only be sustained if both timber and dampness are present. Removal of all timber from contact with an infected damp wall and drying of the wall and the associated timber will therefore prevent further spread of and damage by the fungus.

During the drying process of the infected masonry fruiting–bodies may appear. It is important to appreciate that the fungus may lie dormant in dry timbers, including small fragments within dry walls for a year or so and possibly longer in cooler areas such as basements. If damp conditions are allowed to return the fungus may become active again.

The strands of dry rot fungus can conduct moisture and nutrients to support growth, this ability is dependent on the moisture conditions in the surrounding wood or masonry, the fungus cannot colonise dry, well ventilated wood, or masonry. This ability to conduct moisture allows the fungus to spread from damp timber to adjacent, poorly ventilated voids where it is able to introduce additional water to marginally damp timber which might otherwise remain just below the moisture threshold for decay.

Gloss/vinyl paints and other impervious coating can trap moisture in masonry and stud walls. This will then allow the fungus to introduce sufficient moisture for the fungal mycelium to spread progressively through the coated masonry or timber. By this process the fungus can spread through voids such as those beneath poorly ventilated ground floors or behind painted internal panelling.

Dry rot requires three elements to sustain growth which are:

Water, air and food (Timber-Cellulose)

Treatment

Treatment will tend to differ from property and infections. We highly recommend requesting a specialist survey and investigation to determine remedial action required.

Primary control

Identify and eliminate source of moisture

Promote rapid drying

In some case where drying of saturated masonry can take several years secondary control may be required